Ali RA used to forbid people from criticizing Uthman RA for burning the Qurans and gathering the people around one reading:

Ali RA said: O people do not make Ghulu in Uthman and only say what is good about him, By Allah he did what he did openly in front of all of us(companions) .. by Allah if I were given authority I would do the same as he did.

Ali RA Then said: When the people differed regarding the Quran and this had reached Uthman he then gathered us the companions of the messenger of Allah PBUH and he consulted us about uniting the people on one reading, so our opinion was the same as his, (Ali then continued): “If I was given the authority which he was given I would have done as he did.”

Imam Ahmad narrated with its Isnad from Muhammad Ibn al Hanafiyah RA: Ali was told that Aisha is cursing the killers of Uthman in al Marbad(location near al-Basrah) So he lifted his hands until they reached his face and said: “And I curse the killers of Uthman, may Allah curse them in the valleys and the mountains” he said it two or three times.

source: Fada’el al Sahabah 1/455 #733 and the Muhaqqiq of the book said SAHIH, also in al Riyadh al Nadirah fi Manaqib al ‘Asharah 3/70.

Imam Ahmad with its Isnad from Muhammad Bin Hatib the young Sahabi: I asked Ali about Uthman and he said: “He was amongst those who believed then had Taqwa and then he had (more) belief and then (more) Taqwa”.